Movement of an object during imaging modalities like CT has a negative influence on the quality of the obtainable images. Specifically while taking images of creatures, especially human beings, the quality of images suffers from motion artefacts induced by the beating heart. Therefore, one tries to determine those time periods when there is no such motion, and to do the imaging then. This technique is called gating. Today, the trigger signal for the image acquisition is usually derived from the ECG signal of the patient. Using the ECG signal for gating purposes means looking at the electrical excitation of the heart. Of course, the electrical excitation of the heart is correlated with its mechanical action. However, the reaction time of the heart muscle is unknown and varies. Because of that uncertainty and in order to be on the safe side, the period of time that is predicted to be the next resting phase of the heart has to be shortened by a certain amount of time. In other words, by looking at the ECG for gating purposes, one has to give away valuable data acquisition time simply because ECG is not a precise indicator of the true mechanical action of the heart. If the resting phases of the heart could be estimated better, and if they could thereby be fully exploited for the data acquisition, the total acquisition time could be shortened and the X-ray load imposed on the patient could be reduced. But in order to do that, one has to look directly at the mechanical action of the heart for which different methods are used. It is possible to measure the motion of a patient's heart and chest from a distance with the help of electromagnetic waves, in particular Doppler radar. However, Doppler radar responds to more than the heart motion alone. Because the Doppler radar sensor radiates electromagnetic waves in a broad radiation pattern, all moving parts around the sensor produce artefacts on the sensor signal. In current computer tomography (CT) systems, it is in particular the rotating gantry that produces low-frequency artefacts in the Doppler radar signal. The problem of artefacts due to the body's movement also occurs during MRI imaging. The document US 2003/0195413 A1 (Rubin, Jonathan M. et al.) unveils an MRI system with a detector system used for producing a gating signal for the MRI system. The detector system comprises an ultrasonic transducer for detecting movement of the object while the images are being acquired by the means of the MRI system. In one embodiment an acoustic waveguide is used that extends into the bore of magnet in order to place the transducer outside the field of the MRI system. In WO 02/41776 A1 (Feinberg, David) it is further proposed to use ultrasound to determine the position of the moving organ and to translate the new parameters into an angulation and displacement of the subsequently acquired MRI image volume. But it has shown that with the means of ultrasonic sound the resolution being obtainable for deeper tissue structure is less compared with CT- or MRI-systems. Also, the coupling of the acoustic waves into the body requires an acoustic coupling medium which may result in false signals and which is undesired in respect of user-comfort. Especially for investigating the human heart with the means of ultrasonic sound, only specific so-called ultrasonic windows can be used through which a direct acoustic path to the heart is possible due to the human anatomy. Outside these windows reflections can be caused by the lung or by bone-structures. This means that for ultrasonic investigations of the heart, experience is required to be able to place the transducer properly. Hence there still is a need for a more precise indicator of the actual mechanical action of the heart.